There is considerable interest in heterostructure devices involving greater epitaxial layer thickness and greater lattice misfit than present technology will allow. For example, it has long been recognized that germanium-silicon alloy Ge.sub.x Si.sub.1-x grown on silicon substrates would permit a variety of optoelectronic devices, such as LEDs, marrying the electronic processing technology of silicon VLSI circuits with the optical component technology of direct band semiconductors. Indeed, it has been proposed that an intermediate epitaxial layer of germanium-silicon alloy would permit the epitaxial deposition of gallium arsenide overlying a silicon substrate and thus permit a variety of new optoelectronic devices. However, despite the widely recognized potential advantages of such combined structures and despite substantial efforts to develop them, their practical utility has been limited by high defect densities in heterostructure layers.
A highly advantageous method for making a semiconductor heterostructure of germanium-silicon alloy on silicon is disclosed in U.S. patent application Ser. No. 07/690,429, filed in the names of Brasen et al entitled "Method For Making Low Defect Density Semiconductor Heterostructure and Devices Made Thereby" and assigned to applicants' assignee. The Brasen et al application discloses that one can grow on silicon large area heterostructures of graded Ge.sub.x Si.sub.1-x alloy having a low level of threading dislocation defects by growing the alloy at high temperatures in excess of about 850.degree. C. and increasing the germanium content at a gradient of less than about 25% per micrometer. Using this method one can grow low defect heterolayers of high germanium alloy.
The present invention is directed to the next step toward the long sought goal of direct band optoelectronics on silicon, namely a method for growing a low defect heterolayer of gallium arsenide on a layer of germanium.
Efforts to grow GaAs on group IV semiconductors predominantly begin with the growth of a prelayer of arsenic on the group IV substrate. The use of As prelayers has dominated GaAs/Si experiments because: a) As pre-layers are self-limiting, i.e. only one monolayer will deposit; and b) the As background pressure in most systems is high, so in the absence of special precaution, an As prelayer is unavoidable. Experiments have been conducted using Ga prelayers in GaAs growth on Si but the GaAs growth is equally poor (three-dimensional) using Ga or As prelayers. (See, for example, R. D. Bringans et al, Appl. Phys. Lett. 51, 523 (1987) and M. Zinke-Allmang et al, Surf. Sci. Rep. 16, 446 (1992).
Despite the closer lattice spacing, efforts to grow GaAs on Ge have also been less than satisfactory, with both As prelayers and Ga prelayers producing three dimensional, multiple domain growth. See, for example, S. Strite et al, Appl. Phys. Lett. 56, 244 (1990). Accordingly there is a need for an improved method of making semiconductor heterostructures of gallium arsenide on germanium.